Umair Gaines
An electric discharge is the release and transmission of electricity in an applied electric field through a medium such as a gas. This phenomenon can occur in various forms, including self-sustaining and non-self-sustaining discharges, and has several practical applications. For example, spark gaps in internal combustion engines ignite the fuel/air mixture, and electric arcs are used in welding and steelmaking. Understanding the behaviour of electrical discharges is crucial for harnessing their power and mitigating any potential issues, such as those caused by high-voltage sparking.
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What You'll Learn
Self-sustaining and non-self-sustaining discharges
Electrical discharges have historically been divided into two categories: self-sustaining and non-self-sustaining. The transition between the two is abrupt and occurs through the formation of a spark. Non-self-sustaining discharges occur at relatively low currents (around 10−8A). Townsend discharges, which occur in small gaps without a hot channel, are a particular type of non-self-sustaining discharge. Townsend discharges are induced by irradiating the gas between two electrodes to produce initial ionisation. They are non-sustaining because the current flow stops as soon as the ionising radiation is removed.
Non-self-sustaining discharges are characterised by a loss of charge. Ions and electrons cannot stimulate each other and thus recombine to become neutral. Non-self-sustaining discharges can be controlled by a stationary fast electron beam. The numerical model of a non-self-sustaining discharge (NSSD) considers the influence of the dust component on electron and ion densities.
Self-sustaining discharges, on the other hand, continue even after an external ioniser ceases to operate. They are of two types: glow discharges and arc discharges. Glow discharges are characterised by a nearly independent potential difference across the discharge, extending to at least 10−3 A or several amperes. At higher currents, the voltage increases, forming an "abnormal" glow discharge. The glow discharge is visually characterised by a diffusely luminous plasma extending across the discharge volume.
Arc discharges, on the other hand, require only a low voltage for sustenance and can cause currents from 10−1 A to above 105 A. At higher gas pressures, both the anode and cathode of the arc may reach the boiling temperature of the electrode material. The arc core is typically at temperatures ranging from 5 × 103 to 30 × 103 K, resulting in a highly dissociated and ionised gas.
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Townsend discharges
The Townsend discharge was named after John Sealy Townsend, who discovered the fundamental ionisation mechanism in his work around 1897 at the Cavendish Laboratory in Cambridge. Townsend's early experiments involved planar parallel plates forming two sides of a chamber filled with gas. He connected a direct-current high-voltage source between the plates, with the lower-voltage plate being the cathode and the upper-voltage plate being the anode. He forced the cathode to emit electrons using the photoelectric effect by irradiating it with x-rays. Townsend found that the current flowing through the chamber depended on the electric field between the plates and that the current increased exponentially as the plate gaps became smaller. This led to the conclusion that the gas ions were multiplying as they moved between the plates due to the high electric field.
The Townsend discharge occurs in a gaseous medium that can be ionised, such as air. It requires a source of free electrons and a significant electric field; without both, the phenomenon does not occur. The electric field and the mean free path of the electron must allow free electrons to acquire an energy level (velocity) that can cause impact ionisation. Townsend discharges can be sustained only over a limited range of gas pressure and electric field intensity. At higher pressures, discharges occur more rapidly than the calculated time for ions to traverse the gap between electrodes.
The Townsend avalanche can have a large range of current densities. In common gas-filled tubes, magnitudes of currents flowing during this process can range from about 10−18 to 10−5 amperes. The Townsend avalanche is fundamental to the operation of gaseous ionisation detectors such as the Geiger–Müller tube and the proportional counter, which are used to detect ionising radiation or measure its energy. The Townsend discharge sets the upper limit to the blocking voltage a glow discharge gas-filled tube can withstand. This limit is known as the Townsend discharge breakdown voltage or ignition voltage of the tube.
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Glow and arc discharges
Glow discharge is a type of plasma formed by passing an electric current through a gas, usually argon or another noble gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the striking voltage, the gas ionization becomes self-sustaining, and the tube glows with a coloured light. The colour depends on the gas used.
Glow discharges are used as a source of light in devices such as neon lights, cold cathode fluorescent lamps, and plasma-screen televisions. They are also used in plasma physics and analytical chemistry, as well as in the surface treatment technique called sputtering.
The voltage across a glow discharge consists of two major components: the cathode fall and the positive column. Most of the voltage drop occurs across the cathode fall. The positive ions are driven towards the cathode by the electric potential, and the electrons are driven towards the anode by the same potential. This process is known as secondary electron emission.
An arc discharge is a self-sustained discharge requiring only a low voltage for its sustenance and is capable of causing currents from typically 10−1 A to above 105 A to flow. The current density at the cathode of the arc is greater than that at the glow cathode. The electron emission process for the arc is different from that of the glow and is often thermionic in nature.
The arc core is typically at temperatures ranging from 5 × 103 to 30 × 103 K, while the temperature of the aureole spans from 2 × 103 to 5 × 103 K. Arc discharges are used in arc welding, electric arc furnaces, and the production of alloys and other products.
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Electric arcs
The formation of an electric arc involves the creation of a spark, which then transitions into the arc. This arc can be understood as a self-sustained discharge that only requires a low voltage input to maintain itself. One of the key differences between an arc discharge and a glow discharge is the current density at the cathode. In an arc discharge, the current density is greater, indicating a different electron emission process that is often thermionic in nature.
At atmospheric pressure, an electric arc manifests as a constricted, highly luminous core enveloped by a more diffusely luminous aureole. The core of the arc can reach temperatures ranging from 5 × 103 to 30 × 103 K, resulting in a completely dissociated and highly ionised gas. The aureole, on the other hand, has a lower temperature range of 2 × 103 to 5 × 103 K, which facilitates dissociation and chemical reactions.
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Lightning
An electrical discharge is the release and transmission of electricity in an applied electric field through a medium such as a gas. There are several types of electrical discharges that occur naturally on Earth, one of which is lightning.
The process of lightning involves the formation of a spark or hot channel, through which large currents flow. This channel can be referred to as a leader or a stroke. The high voltage and current associated with lightning can cause a range of effects, including the ionization of the surrounding air, the formation of plasma, and the generation of heat.
The electric arc formed during a lightning strike is a self-sustained discharge that requires only a low voltage but can sustain extremely high currents. The arc is characterized by a constricted, highly luminous core surrounded by a more diffuse aureole. The temperature of the core can reach up to 30,000 K, while the aureole has a lower temperature that facilitates chemical reactions and dissociation.
The occurrence of lightning can have significant impacts on the environment and human activities. For example, lightning can cause fires, damage electrical equipment, and affect the operation of power grids. Additionally, lightning is associated with thunder, which can be loud and potentially harmful to sensitive hearing.
In summary, lightning is a dramatic example of an electrical discharge occurring in nature. It involves the rapid movement of charged particles, resulting in a bright flash and a loud thunderclap. The study of lightning and its effects is crucial for understanding atmospheric electricity and developing measures to protect people, structures, and equipment from its potential hazards.
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